1. Field of the Invention
The present invention relates to universal serial bus system and more particularly relates to termination function of bus for peripheral equipment connected to universal serial bus system.
2. Description of Related Art
“Universal Serial Bus Specification”, a non-patent publication, discloses specifications of universal serial bus, abbreviated as USB hereinafter. The specification describes that in case peripheral equipment is connected to a bus and in case a high-speed mode in which a termination circuit is not connected to a bus shifts to a suspend state, termination operation of a bus is started by a pull-up circuit such as a pull-up resistor or the like as a termination circuit.
When a termination operation starts, a signal line D+ or D− is pulled up and the following matters are verified: (1) whether or not peripheral equipment is connected to a host or a hub; and (2) whether or not a high-speed mode is suspended and operation speed shifts to a full-speed mode.
FIG. 5 shows a termination operation of a USB. A USB 100 is connected between a host or a hub and peripheral equipment. Depending on its wiring length, a parasitic resistor RL and a parasitic capacity CL are provided as wiring load. One of terminals of the USB 100 is connected to a pull-up circuit provided in peripheral equipment (not shown). The pull-up circuit PU1 includes a switch circuit SW1 that connects between pull-up voltage VH and USB 100 via a termination resistor R1. When the switch circuit SW1 is closed, the USB 100 is charged with pull-up voltage VH. Voltage level does not rise suddenly due to a response time of the switch circuit SW1, pull-up drivability of the termination circuit PU1, wiring load on the USB 100, and the like, but reaches its predetermined voltage through predetermined propagation delay. Accordingly, predetermined time is required for a host or a hub to detect a termination operation by peripheral equipment.
When the host or the hub verifies a termination operation by peripheral equipment, the operation is set to SE0 (Signal Ended 0) state to set signal lines D+ and D− in negate state. When SE0 state lasts for 2.5 μsec or longer, bus reset state is transmitted and peripheral equipment is initialized. In addition to it, data-transfer speed is determined. Thereby, data transfer between the host or the hub and peripheral equipment becomes possible.
However, the signal lines D+ and D− are in negate state until the termination circuit PU1 starts operation from a termination start instructed by peripheral equipment and either the signal line D+ or the signal line D− is charged up to predetermined voltage level. Accordingly, there is a fear that the peripheral equipment erroneously detects the above period as SE0 state. In case this erroneous detection period lasts for 2.5 μsec or longer due to a response time of the switch circuit SW1, pull-up drivability of the pull-up circuit PU1 or wiring load on the USB 100 and the like, there arises a fear that the peripheral equipment erroneously detects a bus reset state, which is problematic.
That is, between the peripheral equipment that has erroneously detected a bus reset state and the host or hub that has not yet detected the signal line D+ or D− being pulled up, the USB 100 has inconsistent recognition state at its interface. Such inconsistent recognition state can possibly cause operation trouble afterwards, which is problematic.
For example, in case erroneous detection of SE0 state lasts for 2.5 μsec or longer, peripheral equipment erroneously can detect a bus reset state erroneously to solely initiate bus reset sequence irrespective of an instruction from a host or the like. An aspect that a signal line D+ is pulled up as termination operation is taken into consideration, a signal line D− is asserted based on specified sequence in case peripheral equipment shifts to chirp processing when the signal line D+ reaches a predetermined voltage level. However, due to the above operation, a host or the like detects both the signal line D+ and the signal line D− as being asserted. Thereby, the host or the like detects disorder of connected peripheral equipment or a bus and connection to the bus is shut down. Shut-down of connection to a bus hinders normal operation of system, which is problematic.